Heretofore, secondary batteries have been widely used, such as lithium secondary batteries, etc. A lithium secondary battery typically comprises positive and negative electrodes comprising positive and negative electrode active materials capable of storing and releasing lithium ions, respectively, a separator to electrically insulate the positive electrode and negative electrode, and a non-aqueous electrolyte impregnated in the positive electrode, negative electrode and separator. In producing a lithium secondary battery, a positive electrode and a negative electrode are layered via a separator to constitute an electrode assembly; the electrode assembly is placed in a battery case and impregnated with a non-aqueous electrolyte to construct a secondary battery; and the secondary battery is subjected to initial conditioning charging.
In producing the secondary battery, it is known that a metal contaminant such as iron (Fe) can be inevitably mixed in from the outside. When the metal contaminant is present on or near the positive electrode, the metal contaminant dissolves into the non-aqueous electrolyte during charging of the secondary battery (e.g. during initial conditioning and during the use), and precipitate out gradually and locally in the opposite area on the negative electrode. It has been reported that if the metal contaminant is large in size, the precipitates will pierce through the separator to cause a short circuit.
Accordingly, it has been suggested that after a secondary battery is constructed, but before it is subjected to initial conditioning charging, the secondary battery be subjected to a process to prevent a short circuit caused by such a metal contaminant (or “metal contaminant deactivation (harm minimization, risk control) process” hereinafter) (e.g. see Patent Document 1).
For instance, Patent Document 1 discloses a secondary battery production method in which a secondary battery is initially charged to 0.01% to 0.1% of the secondary battery capacity and then left standing for 1 hour to 48 hours. It is disclosed that according to such a method, by the initial charging, dissolved metal ions are diffused in the electrolyte solution, allowing prevention of local precipitation of the metal on the negative electrode during subsequent charges.